The objective of this research is to determine the range of variation of certain phenomena of heart rate, rhythm and conduction which occur in the middle-aged men under the conditions of daily life, and the relation of these phenomena to coronary heart disease and to sudden death. In 1962 a sample of 20,000 to 30,000 cardiac complexes was obtained from each man in a sample of 301 actively employed men, aged 55 by recording his electrocardiogram over a 6 to 7 hour period of carefully standardized activities. The men were also studied in terms of various medical, physical, biochemical, behavioral and environmental factors thought to be relevant to coronary heart disease. They were followed prospectively for five years; all deaths and illnesses were investigated; they were re-examined; and 24-hour recordings of approximately 120,000 complexes made under standard conditions of position, activity, food intake and sleep were obtained from 214 of the 261 survivors. In 1966-67, 127 men, aged 30 to 40, were similarly examined, and six-hour recordins were obtained on a comparable routine. Since 1970, 400 additional men, aged 40 to 65, including 300 with known CV disease and at high risk of death, have been examined similarly and are being re-examined three times a year. The findings indicate that asymptomatic dysrhythmias, disorders of conduction and disorders of the pacemaker are quite common in middle-aged American men, progress with increasing age, and are associated with ischemic heart disease, hypertensive heart disease, chronic obstructive pulmonary disease, and alcoholism. They appear to be significant precursors of sudden death. Funds have been requested for three years to continue the analyses of the data that have been accumulated relating to these findings, to publish the findings and to carry out follow up studies of 127 men now in their late 40s who were first studied in their late 20s.